Is there any other way to download iOS 6 into the computer, so that i can update it later on? I don't want to wait 3-4 hrs updating my iPhone. abt 650mb file is pretty big to download in my case. Also since there is irregular interuption with my net, i cannot trust to let my iTune syc it within 3-4 hrs. I don't think that i can update half the file today, and rest later on from iTune either.
Since you have an iPhone 4S, it must have at least iOS 5 already. Therefore, you can update its iOS over-the-air directly without any computer. Simply do Settings > General > Software Update on your iPhone.